Spectacle lenses are grouped as those having one refractive power and those having several refractive powers but at least two different refractive powers. The different refractive powers of the last-mentioned spectacle lenses enable the viewer to see without difficulty in at least the near and far ranges.
The multifocal surface of these spectacle lenses is the surface which generates different refractive powers in different zones of the lens and thereby makes viewing possible without difficulty for the far and near ranges and possibly also for intermediate ranges. The multifocal surface can be configured as a bifocal surface, trifocal surface or a multifocal surface.
The prescription surface is the opposite-lying surface adapted to this multifocal surface so that the spectacle lens has the required dioptric powers at the reference points. Dioptric power is the generic term for spherical, astigmatic and prismatic action for a specific ray direction at a specific point in the spectacle lens (especially the reference point for the far-vision zone or the reference point for the near-vision zone).
Spectacle lenses exemplary of the foregoing are the multifocal lenses disclosed in U.S. Pat. Nos. 4,606,622 and 5,137,343 and German Patent 2,044,639. Additional multifocal lenses are described in the article of G. Guilino et al entitled "Neue progressive Flachen" published in the journal DOZ, Number 11, Nov. 20, 1980, pages 20 to 23.
The suitability of a spectacle lens for a specific spectacles wearer is dependent upon the individual use situation. This use situation arises from the individual use requirements which include dioptric power, corneal vertex distance, object distances, forward inclination of the spectacle frame et cetera.
British Patent 1,239,620 discloses an ophthalmic lens having a greater refractive power and pregiven astigmatic effect for a first surface which is spherical or toric and which has a second surface which is corrected with reference to field curvature and deviations of the astigmatism from the constant. It is not possible with this lens to additionally consider individual use requirements because of the symmetry requirements and because the necessary degrees of freedom are no longer available.
The finished spectacle lens is produced in a manufacturing process by applying a spherical or toric prescription surface to the so-called semi-finished piece which already has a multifocal surface. The prescription surface is necessary for the required dioptric power. This multifocal surface is so conceived that the imaging errors for the wearer of the spectacles correspond to the desired "design" over the entire lens for a specific average use situation in combination with a spherical or toric prescription surface corresponding to this use situation. The distribution of the average power and of the astigmatic deviation and/or of the horizontal or vertical prismatic action and/or the distortion is characterized as the "design" of the multifocal surface. These distributions in the average use situation are characterized in the following as the optimized basic case.
For reasons of cost, this semi-finished piece is utilized for a wide range of use situations. An individual spherical or toric prescription surface is applied to the semi-finished piece in order to obtain the dioptric power for the particular spectacles wearer. In this way, the usable zones for viewing far and for viewing near and for viewing at intermediate distances are limited.